A switched reluctance machine is a brushless, synchronous machine having salient rotor and stator poles. There is a concentrated winding on each of the stator poles, but no windings or permanent magnets on the rotor. Each pair of diametrically opposite stator pole windings is connected in series or in parallel to form an independent machine phase winding of the multi-phase switched reluctance machine. Ideally, the flux entering the rotor from one stator pole balances the flux leaving the rotor from the diametrically opposite stator pole, so that there is no mutual magnetic coupling among the phases.
Torque is produced by switching current in each phase winding in a predetermined sequence that is synchronized with angular position of the rotor. As a result, there is a magnetic force of attraction between the rotor poles and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction would produce a negative or braking torque. Hence, by properly positioning the firing pulses relative to the rotor angle, forward or reverse operation and motoring or generating operation can be obtained.
In practice, a switched reluctance generator operating open loop in the full square mode (i.e., such that the inverter switches driving each phase turn on and off once per cycle) is typically unstable. In particular, small perturbations in the dc bus voltage cause the average generated current to increase which, in turn, causes the dc bus voltage to increase further, resulting in an unstable drive. Furthermore, even if feedback is used to close the control loop, the system response to step load and commanded voltage changes depends on the dc bus voltage and is thus not optimum.
Accordingly, it is desirable to stabilize open loop operation of a switched reluctance generator as well as to improve the closed loop performance thereof.